I agree with the different levels of consciousness, I have awoken on the surgery table, sat up and pulled the breathing tube out, because I am a redhead and apparently we need more anesthesia to stay under.
My last suicide attempt, which was over about 17 years ago, I ground two full large bottle of benedryl up and mixed them with rootbeer, did not throw up, went to bed and awoke to music playing, (in this apartment, the phone rang and no one was there, lights turned themselves on and off, and the stereo turned itself on and off all the time and would even change stations on it's own, but I had unplugged it before I took the overdose, it was still unplugged, but music was still playing from the speakers), I remember partying with the fairies, that lived in my apartment, I served them butter, they like butter. Then, my now father in law, who, at the time I had never met, came and danced with me and told me to hold on, his son needed me. I remember the paramedics being there, have no idea how they got in, I could not have let them in as I had thrown the keys out the window prior to taking the od, I heard them talking on the radio that my pupils had no response, that I had no pain response when they place an IV in my arm, but I was still breathing shallow breaths on my own. I was totally aware of everything they were doing to me, but I could not respond and could not get my body to respond to my desire to get away from them, I was aware of everything, but was not able to respond for almost 24 hours.
So yes, there ARE different levels of consciousness and yes, you CAN be fully aware of what is happening to you and be completely unable to respond in any way to even let anyone know you are in there.
THIS, is why it has been over 17 years since my last attempt.